Depends on the ferry ride. River crossings like Mississippi River is a relatively short ride and there is no place to go. Likewise Champlain Ferry (not the Burlington one) is all business. they really don't want you wandering around as the ride is only 20 minutes. Several crossings we have taken are only 5 minutes.
Long haul ferries like the Old Portland ME to Yarmouth NS no one was allowed on the car deck at all during the 11 hour passage. Likewise on the Alaska Marine Highway.
AS for disconnecting the tow'd If I had wanted to pay the perfoot rate for the distance from the coach back bumper to the tow'd front bumper, I was free to do so. Not only did I disconnect the tow'd, I dismounted the towbar as it extended 2 feet from the coach body.